There is an increasing demand on local telephone companies to provide more communication channels at each customer premises. In an individual home, for example, there may be a need for two or more communication channels to carry voice data and one or more channels to support digital communications for such devices as a facsimile machine, a personal computer, or an internet terminal. Various equipment exists today to enable multiple communication signals to be conveyed on a single twisted paired telephone transmission line such as digital added main line (DAML) Systems, basic rate integrated services digital network (ISDN) systems, circuit multiplexers, and some implementations of IP telephony wherein voice signals are conveyed via Internet Protocol Packets routed through the Internet. In such existing systems, all of the communication channels are terminated by one transceiver-multiplexer at each end of the telephone transmission line, and all of the communication channels are combined by a common unit and transmitted as one modulated signal.
Recently, twisted pair telephone transmission line connections have been used for communicating two simultaneous channels such as digital data and analog voice signals. Typically, a high speed digital subscriber line (DSL) channel such as ADSL and a plain old telephone system (POTS) channel are established over a single twisted pair wire connection. A POTS splitter is typically utilized to decouple the channels into separate frequency bands. The POTS channel usually resides in a frequency spectrum of about 0 kHz to about 4 kHz, and the ADSL channel resides in a frequency spectrum of about 20 kHz to about 500 kHz. A low pass filter is often included in such a system to isolate the channels and minimize high frequency transients produced by on-hook/off-hook transitions which can degrade the high speed data transmission on the ADSL channel.
FIGS. 1A and 1B show one implementation of a DAML system just described. In FIG. 1A, the customer premises 10 is connected to the public switched telephone network 12 through the twisted pair transmission line 14 connected into the main distribution frame 16. DAML unit 20 is connected to the network interface device 18 through wall jack 19. The DAML unit 20 supports two independent communication channels 22, 24 by multiplexing the signals and transmitting them across transmission line 14 as a single modulated signal. Phone 1 communicates on baseband POTS. A low pass filter (LPF) 21 isolates the higher frequency transients and interference between the two communication channels. A corresponding DAML unit 13 and LPF 15 are connected on the network side of the system. FIG. 1B represents the frequency band of the signal transmitted across transmission line 14. Phone 1 communication is baseband POTS 23 and phones 2 and 3 communicate in a combined, higher frequency channel 25. Most DAML systems currently omit the baseband POTS channel.
Traditional DAML systems are designed to work on nearly all customer transmission loops. Since transmission signal quality is related to the customer distance from the central office, the number of communication channels a DAML system can support is limited by the worst-case scenario transmission loop in the overall system. In other words, the DAML system must be able to support the same number of additional communication lines for customers furthest from the central office as it does for customers nearest to the central office. Accordingly, the upper frequency range supported by the DAML units is artificially limited for customers whose transmission loops would support higher frequency ranges and, therefore, additional communication channels.
The present invention overcomes this drawback by deriving additional communication channels wherein each additional communications channel is modulated into a separate signal in a separate frequency band by way of a separate transceiver unit such as a DAML. For each additional communications channel desired, a separate transceiver unit is connected to the telephone transmission line at the customer premises in, for example, a wall jack. Each transceiver unit automatically utilizes the lowest unoccupied frequency band by monitoring each frequency band for the presence of signal power. This configuration enables transmission lines of customers closer to the central office to support several derived communications channels, whereas transmission lines for customers further from the central office with less usable bandwidth could still be used to support a fewer number of derived communications channels.